The present invention relates to an automatic symmetry correction circuit for a symmetrical current chopper. It applies to the construction of static DC converters and regulators having current choppers with transformers.
Among the known constructions of DC current converters or regulators with a transformer, the symmetrical circuits have at least three advantages as compared with asymmetric circuits. These advantages may be summed up in the following way:
1. They use the entire hysteresis cycle of the core of the transformer. This allows the size of this latter to be reduced, as well as its price for comparable output frequency, efficiency and power
2. The pulsed currents called for and delivered have a frequency double the operating frequency, which facilitates filtering.
3. The cyclic ratio, i.e. the ratio of the energy transfer time to the duration of a cycle may be very close to unity, which reduces the efficient currents present in all the power components.
Although symmetrical circuits have been interesting from a theoretical point of view, there are many problems in actually constructing one. They generally require a large number of components which the asymmetric circuits do not. Their control circuit is more complex. The risks of simultaneous conduction in the two conduction channels and the maintenance of perfect operating symmetry between the two channels lead to further difficulties of construction.
The causes of operating asymmetry are numerous. Among them exist those due to the differences of characteristics of two symmetrically connected components, those resulting from a symmetry of wiring or arrangement of the integrated circuits on the printed circuit, those resulting from a difference in length of the control square waves of two switches due, for example, to an imperfection of the control circuit or to the reintroduction into the sensitive stages of the control circuit of a part of the voltage square wave or of the current saw tooth wave present in the primary and the secondary of the transformer, introduced through non symmetrical capacitive or inductive parasite connections.
Asymmetry of operation results in a shift of the operating point of the transformer with respect to its magnetic dynamics, reduction of the energy efficiency and a reduction of the reliability. It leads to over dimensioning the power components and finally it is the cause of risks of instability.